1. Field of the Invention
The invention relates to a hybrid vehicle and, more particularly, to a hybrid vehicle including first and second rotary electric machines and a transmission unit.
2. Description of Related Art
There is known a hybrid vehicle including not only an engine, two rotary electric machines and a power split mechanism but also a transmission mechanism between the engine and the power split mechanism.
A hybrid vehicle described in International Application Publication No. 2013/114594 employs a series-parallel hybrid system. In the vehicle having a series-parallel hybrid system, the power of an engine is transmitted to a first rotary electric machine (first motor generator) and is used to generate electric power, while part of the power of the engine is also transmitted to drive wheels via a power split mechanism.
There is also known a hybrid vehicle having a configuration (series hybrid system) by which the hybrid vehicle generates electric power by the use of the power of an engine and travels in series mode in which a motor is driven by the generated electric power. In this series hybrid system, the power of the engine is not transmitted to drive wheels.
The hybrid vehicle described in International Application Publication No. 2013/114594 is not able to travel in series mode because the power of the engine is also transmitted to the drive wheels via the power split mechanism at the time when the power of the engine is transmitted to the first motor generator.
In the series-parallel hybrid system, there is a concern that tooth contact noise occurs in a gear mechanism provided in a drive system between the engine and the drive wheels because of torque fluctuations of the engine at a low vehicle speed, or the like, so it is required to select an operating point of the engine such that tooth contact noise does not occur, and the engine may be operated at an operating point at which fuel consumption is not optimal. Therefore, there is room for improvement in fuel consumption.
On the other hand, in the series hybrid system, the engine is completely isolated from a gear mechanism provided in a drive system, so such tooth contact noise does not need to be considered so much. However, the entire torque of the engine is once converted to electric power and then the electric power is converted back to the torque of the drive wheels with the motor, so the series hybrid system is inferior in fuel consumption in a speed range in which the operation efficiency of the engine is high to the series-parallel hybrid system.
In this way, there is a point in which the series hybrid system is superior to the series-parallel hybrid system and there is also a point in which the series-parallel hybrid system is superior to the series hybrid system, so it is desirable to be configured to allow one of the series mode and the series-parallel mode to be selected in response to the condition of a vehicle.
When such a hybrid vehicle that is able to perform both the series mode and the series-parallel mode is implemented, if the engine is started by the first motor generator while the power transmission unit (transmission unit) is placed in a non-neutral state where the output from the engine is transmittable to a side downstream of the power transmission unit as is performed in the vehicle described in International Application Publication No. 2013/114594, an engine startup shock may be transmitted to an output shaft coupled to wheels and a shock that is experienced by a driver may increase.